1. Field of the Invention
The present invention relates to a hydraulic active suspension system for a vehicle such as an automobile or the like, and, more particularly, relates to a hydraulic active suspension system which is improved to enhance the comfortability of a vehicle while effectively controlling the attitude change of a vehicle body.
2. Prior Art
As described, for example, in Japanese Patent Laying Open Publication No. 62-187609 (1987), as one of active suspension systems for a vehicle such as an automobile or the like, a hydraulic active suspension system has already been known which comprises a plurality of hydraulic actuators each being provided corresponding to vehicle wheels and adapted to increase or decrease the vehicle height in response to the hydraulic pressure within its working fluid chamber, and a plurality of control means each being provided corresponding to the actuators and adapted to control the pressure within the working fluid chamber in each actuator to a desired pressure which is determined based upon the difference between a desired and an actual vehicle heights. According to the hydraulic active suspension system of this type, the attitude change of the vehicle body can be performed in a more effective manner as compared with the system in which no feedback control is conducted based upon the vehicle height difference.
In such a hydraulic active suspension system as mentioned above, however, in which the actuator and the control means for controlling the hydraulic pressure within the associated actuator are provided for each vehicle wheel, and the feedback control based upon the vehicle height difference is carried out separately for each vehicle wheel, if the feedback controls are performed to a larger extent so as effectively to reduce the attitude change of the vehicle body such as roll and pitch, the desired pressures may considerably be varied in response to the change in vehicle height due to the undulations of a road surface. In contrast with this, if the effectiveness of the feedback controls is reduced so as to ensure the comfortability of the vehicle, the hydraulic pressure within each actuator may not always be effectively controlled and the attitude change of the vehicle body may not desirably be controlled.